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    <title>Document</title>
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    <script>
        function foo(arr) {
            let reduce = 0;
            arr.forEach(item => {
                reduce = reduce ^ item
            })
            return reduce
        }
        let arr = [1, 2, 5, 2, 1, 4, 4]
        console.log(foo(arr))
    </script>
</body>

</html>